A Syrian air strike killed more than 40 people in a rebel-held area near Damascus, the Syrian Observatory for Human Rights monitoring group and opposition activists said.
Syrian government officials were not immediately available to comment on the reported attack in Hamoria, a district in the Eastern Ghouta area that has been besieged by security forces.
Footage posted on YouTube by the opposition Syrian Media Organization showed several bodies lying on a blood-stained floor, some of them children with blast wounds. Other bodies lay under white shrouds or jackets and a caption said more than 30 people had been killed.
The Observatory, which monitors the war in Syria, said 42 people had been killed, including six children, in what it described as a massacre.
The opposition grassroots Local Coordination Committees said on its Facebook page that the strike hit a public square where people were leaving a mosque after Friday prayers.
Another opposition group, the Shaam News Network, published a video on its Facebook page showing what it said was the aftermath of a blast in an open square. Bodies lay on the road and buildings were damaged.
Reuters could not immediately verify the reports. An employee at the information ministry in Damascus said there would be no one to answer questions until Saturday.
Syria's air force has launched daily air strikes in the civil war, which started after security forces cracked down on pro-democracy protests in 2011.
On Tuesday, an air raid on a cattle market in territory controlled by the hardline Islamic State group killed 43 people, the Observatory monitor said.
Nasir Haj Mansour, an official in the Kurdish administration that controls nearby areas, said the death toll was much higher, around 70.
"The strike, according to our information, was painful for Daesh, in addition to civilians," he told Reuters by Skype, using a derogatory Arabic acronym for Islamic State, which is fighting the governments of Syria and neighbouring Iraq.
The United Nations says 200,000 people have been killed in Syria's war.